1. Field of the Invention
The present invention concerns a method for fastening of sheets made of bitumen or similar corresponding materials to underlying support structure. The invention also relates to a roof construction and to a screw therefore.
2. Description of Related Art
Corrugated metal sheets or corrugated sheets made of plastic or cement on roofs are typically low cost solutions and have corresponding popularity. Such corrugated sheets are typically fastened by screws. French patent document FR2564916 discloses such fastening, where one type of screw is disclosed for wood constructions, where the screw has a winding that extends to the tip, and another screw is disclosed for metal constructions, where the screw has a drill point at the tip. The screw has a head and a washer for tightening against the sheet. An alternative washer is found in European patent application EP0058211.
The above mentioned French patent document FR2564916 discloses two screws, one for wood and one for metal. The drill screw in French patent document FR2564916 is not useful for wood construction, because the drill point has a diameter larger than the stem of the screw, such that the winding would not have sufficient power to hold the screw in wood. Both of the screws are provided with wings for increasing the hole diameter of the sheet. This is so because plastic sheets expand and contract substantially on temperature shifts and also have a tendency to move on the roof, and the hole should tolerate such movement.
An improvement in screws for fastening plastic or metal sheets is found in Danish utility model DK200600177U3, which discloses a screw having a tip with a drill that has a reduced diameter relatively to the diameter of the screw stem. The idea behind such construction is that the drill is stable enough to drill through thin steel constructions but not so large that it compromises stability when screwing into wood construction.
Thus, the technology of fastening corrugated sheets of metal, cement or plastic onto roof constructions has involved remarkable development of screws.
Similar considerations on fastening improvements have not been found in connection with corrugated Bitumen sheets, which are nailed into the roof construction. This appears also clearly from the instructions by one of the largest producer of Bitumen sheets, LINALUX; an instruction manual is published on the Internet site www.linalux.com, and part of it is reproduced in FIG. 9. This is so, despite the fact that across the world, millions of corrugated Bitumen sheets are each year being used as roofing material in private housing, industrial building projects, for storage facilities, ad garden houses.
In connection with bitumen sheets, it should be mentioned that the screws in French patent document FR2564916 have wings for increasing the hole through the sheet, which is not useful for bitumen, because the bitumen would not tighten against the screw stem.
Thus, traditionally, corrugated bitumen sheets have been fastened to wood substructures with nails with various non-metallic heads (for example PVC) where the deformation of each said head on top of the corrugation acts as the only means of sealing against rain.
The fastening with nails is time consuming and is not in line with the more modern method of fastening with threaded screw products. As it is recommended that each corrugation on said sheets is fastened to the substructure individually, it means that the extra time consumed using nails is substantial.
Furthermore fastening with nails has the major disadvantage that it lacks precision and cannot be reversed if needed. If during the fastening of the above said sheets the nail goes a little too far into the substructure, it poses a very real problem—as it cannot be pulled backwards without destroying the nail and in some instances both substructure and sheet.
The non-metallic heads mentioned above have historically proven to be problematic. UV radiation from the sun has discolored the heads and/or made them brittle—ultimately causing heads to simply disappear and/or be so damaged they do no longer perform the functions for which they were intended.
Further to the pure functional purposes, there are also several esthetic problems with the above mentioned non-metallic heads. They are typically quite large in order to resist the hammering and this means they are very visible on the roof, which is undesirable.
It is easy to change the color of the Bitumen sheets, whenever a special requirement comes along. However with the solution with non-metallic heads, like plastic molded PVC heads, this presents a logistical challenge, as special color PVC granulate will have to be bought and molded, which is typically something that requires large volumes and very long lead times.
Optimal sealing has always been a known problem with previous solutions where the deformation of the head against the corrugation itself is the only means of sealing. Therefore, unsuccessful attempts have been made by sealing with traditional neoprene, EPDM, and/or similar materials that are normally widely used in the building industry. The main problem however is that neoprene is not very UV resistant and that EPDM is slowly dissolved when it is in contact with oil based products, such as bitumen.
As it appears from above, there is a need for improvements on the art.